This invention relates generally to processes for manufacturing nuclear radiation shields containing boron carbide (B.sub.4 C), and more particularly to methods by which plates and cylinders containing boron carbide particles embedded in a copper matrix can be economically manufactured. The primary use for such shield structures is in the fabrication of containers designed for storage, disposal or transportation of nuclear waste materials and other radioactive substances. One of the known types of containers for nuclear waste materials comprises a plurality of cube-shaped boxes about 9" on a side. The 2-5 mm thick walls made of copper-boron carbide composites contain 20-50% boron carbide by weight. The boxes are embedded in aluminium which is poured (molten) around them and allowed to cool forming a cellular structure.
Boron carbide is the filler of choice because of its high capture cross-section for neutrons. However, absorption of neutrons by boron carbide produces heat. Copper is chosen for the matrix in which the boron carbide particles reside because copper's high specific heat and high thermal conductivity enables it to dissipate a large amount of heat with relatively low temperature rise. Aluminum, in comparison, is not as favorable and has a lower melting point. It is not desirable to use aluminum alone.
The ideal boron carbide-filled copper plate material for use in fabricating these and other types of containers would be a substantially pure voidless matrix of copper metal tightly bonded to a uniformly dispersed boron carbide phase consisting of boron carbide particles arranged within the copper matrix such that no straight line passing through the plate fails to impinge upon a carbide particle. If there is too little copper, a product with voids and diminished structural integrity results. With too much copper the boron carbide particles are too sparsely distributed.
The different properties of boron carbide and copper present problems in fabricating boron carbide-filled copper. One process for manufacturing composite plates involving several separate procedures is described in U.S. Pat. No. 4,227,928 entitled "Copper-Boron Carbide Composite and Method for Its Production", issued Oct. 14, 1980, by C. C. Wang and assigned to the assignee of the present application. In one embodiment of the process, a film of electroless copper is bonded to the boron carbide. Next a relatively thick electrodeposited copper layer is applied to the film. Finally, the copper encapsulated particles, referred to herein as "nodules", are thermo-mechanically consolidated to produce shield structures by hot rolling or hot pressing, with or without sintering, with a copper to boron carbide volume ratio of 0.3-4.0, typically 1.0.
Boron carbide is commercially available in various particle sizes, for example, from the Carborundum Company of Niagra Falls, N.Y. The electrical resistivity of this material is on the order of 10.sup.4 to 10.sup.8 micro-ohms per centimeter. Electrodeposition does not usually lend itself to coating nonconductive materials.